1. Field of the Invention
The present invention relates to multilayer electronic components and methods for manufacturing the multilayer electronic components. The present invention particularly relates to a multilayer electronic component including a laminate and external electrodes formed on the laminate by plating and a method for manufacturing the multilayer electronic component.
2. Description of the Related Art
FIG. 4 shows a multilayer electronic component 101 exemplified by a monolithic ceramic capacitor. The multilayer electronic component 101 includes a laminate 105 including a plurality of stacked insulating layers 102 and a plurality of laminar internal electrodes 103 and 104 disposed between the insulating layers 102. End portions of the internal electrodes 103 are exposed at an end surface 106 of the laminate 105 and end portions of the internal electrodes 104 are exposed at an end surface 107 of the laminate 105. An external electrode 108 is disposed on the end surface 106 of the laminate 105 so as to electrically connect the exposed end portions of the internal electrodes 103 to each other. An external electrode 109 is disposed on the end surface 107 of the laminate 105 so as to electrically connect the exposed end portions of the internal electrodes 104 to each other.
The external electrodes 108 and 109 are formed as described below. A metal paste containing a metal component and a glass component is applied to the end surfaces 106 and 107 of the laminate 105 and is then baked, whereby paste electrodes 110 are formed. First plating layers 111 made of, for example, Ni, are provided on the paste electrodes 110. Second plating layers 112 made of, for example, Sn, are provided on the first plating layers 111. Therefore, the external electrodes 108 and 109 each have a three-layer structure including a corresponding one of the paste electrodes 110, a corresponding one of the first plating layers 111, and a corresponding one of the second plating layers 112.
The external electrodes 108 and 109 must have good wettability to solder because the multilayer electronic component 101 is soldered to a substrate. Since the internal electrodes 103 are electrically insulated from each other, the external electrode 108 must electrically connect the internal electrodes 103 to each other. Since the internal electrodes 104 are electrically insulated from each other, the external electrode 109 must electrically connect the internal electrodes 104 to each other. The second plating layers 112 have wettability to solder. The paste electrode 110 of the external electrode 108 has a function of electrically connecting the internal electrodes 103 to each other. The paste electrode 110 of the external electrode 109 has a function of electrically connecting the internal electrodes 104 to each other. The first plating layers 111 have a function of preventing solder erosion during soldering.
The paste electrodes 110 have a large thickness of several tens to several hundreds of micrometers. Therefore, in order to allow the multilayer electronic component 101 to have predetermined standard dimensions, the multilayer electronic component 101 undesirably needs to have a reduced effective volume because the volume of each paste electrode 110 is relatively large, and the capacitance of the multilayer electronic component 101 depends on the effective volume thereof. If the external electrodes 108 and 109 include only the first and second plating layers 111 and 112, the multilayer electronic component 101 can have a large effective volume because the first and second plating layers 111 and 112 have a thickness of several micrometers.
For example, Japanese Unexamined Patent Application Publication No. 63-169014 discloses a method for depositing conductive metal layers over side surfaces of a laminate by electroless plating such that internal electrodes exposed at the side surfaces thereof are short-circuited with the conductive metal layers.
The method disclosed in Japanese Unexamined Patent Application Publication No. 63-169014 has a problem in that moisture is likely to penetrate the laminate because the bonding of the conductive metal layers to the internal electrodes is insufficient.
WO 2008/059666 discloses a method of overcoming the above-described problem. In this method, plating layers to be converted into external electrodes are formed on end surfaces of a laminate and are then heat-treated at a temperature of about 600° C. or higher and an oxygen partial pressure of about 5 Pa or less, whereby interdiffusion layers are formed between the plating layers and internal electrodes. In the interdiffusion layers, the volume expansion of a metal included therein occurs. Thus, gaps that may be present between insulating layers and the internal electrodes or the external electrodes can be filled.
The interdiffusion layers extend between the internal electrodes and the plating layers. The distance from each of the ends of the interdiffusion layers that face the internal electrodes to a corresponding one of the end surfaces of the laminate is important. WO 2008/059666 describes that the ends of the interdiffusion layers that face the internal electrodes are preferably located at a location about 2 μm or more apart from a corresponding one of the end surfaces of the laminate.
When the interdiffusion layers extend to the position about 2 μm or more apart from a corresponding one of the end surfaces of the laminate, a metal component excessively transfers from the plating layers to the internal electrodes. This reduces the continuity of the plating layers. When the interdiffusion layers extend to a position about 0.4 μm or less apart from a corresponding one of the end surfaces of the laminate and are relatively short, the electrical connection between the internal electrodes and the plating layers is insufficient.